1. Field of the Invention
The present invention relates to a hydraulic circuit for a backhoe vehicle: comprising; a first hydraulic pump, a second hydraulic pump and a third hydraulic pump all driven by a same engine; a pair of right and left vehicle propelling operating valves, with one of the valve pair and an arm operating valve being connected with the first hydraulic pump via a first oil feed passage, the other of the valve pair, a boom operating valve and a bucket operating valve being connected with the second hydraulic pump via a second oil feed passage; and a swivel operating valve connected with the third hydraulic pump.
2. Description of the Prior Art
A backhoe vehicle of the above-described type is known e.g. from a Japanese utility model laid open under Showa 62-31166, in which the relief pressure acting on the oil feed passage extending from the first hydraulic pump and the oil feed passage extending from the second hydraulic pump is fixedly maintained.
However, the above prior art fails to fully utilize the engine power for propelling of vehicle or excavating operation. More particularly, in the above prior backhoe vehicle, considering a situation in which an actuator connected with the first hydraulic pump, an actuator connected with the second hydraulic pump and an actuator connected with the third hydraulic pump are driven at the same time, the engine power is so set as to prevent construction of an engine stop even when the oil feed pressures from all of the first through third hydraulic pumps have reached the relief pressure. In a vehicle propelling or excavating operation, the oil fed from the first and second hydraulic pumps is used for driving the actuators and thus the engine need provide a large load for driving the first and second hydraulic pumps while no such large load is required from the engine for driving the third hydraulic pump. For this reason, the total engine load tends to be smaller than the allowable maximum value. That is to say, the engine power is not fully utilized in the vehicle propelling or excavating operation.
The primary object of the present invention is to provide a construction which makes it possible to make most of the engine power even when the actuators are driven solely by the first and second hydraulic pumps and also to form the construction very simple and easy to assemble.